POJ 2318 计算几何 叉积水题

so water....

/*************************
 * Creater:Sevenster     *
 * Time:2012.07.31 15:14 *
 * PID:POJ 2318          *
 *************************/
 #include<iostream>
 #include<stdio.h>
using namespace std;

class CPoint
{
public:
    double x, y;
};

class CVector
{
public:
    double x, y;
};
/**
CVector CPoint::operator -( CPoint b, CPoint a )
{
    return CVector( b.x- a.x, b.y- a.y );
}

double operator ^( CVector p, CVector q )
{
    return p.x* q.y- q.x* p.y;
}
*/
double xmult( double x1,double y1,double x2,double y2 )
{
       return x1* y2- x2* y1;
}
int main()
{
    int n, m;bool flag=0;
    double x1, y1, x2, y2;
    while( scanf( "%d", &n )!= EOF )
    {
        if( flag==true )
            printf( "\n" );
        flag=true;
        
        if( n==0 )
            break;
        scanf( "%d%lf%lf%lf%lf", &m, &x1, &y1, &x2, &y2 );
        CPoint up[5555],down[5555],toys[5555];
        int cnt[5555];
        memset( cnt,0,sizeof(cnt) );
        for( int i=1;i<=n;i++ )
        {
            up[i].y=y1;
            down[i].y=y2;
            scanf( "%lf%lf", &up[i].x, &down[i].x );
        }
        up[0].x= x1; up[0].y= y1;
        down[0].x= x1; down[0].y= y2;
        up[n+1].x= x2; up[n+1].y= y1;
        down[n+1].x= x2; down[n+1].y= y2;
        for( int i=0;i<m;i++ )
            scanf( "%lf%lf", &toys[i].x, &toys[i].y );

        for( int i=0;i<m;i++ )
        for( int j=1;j<=n+1;j++ )
        {
            if( xmult(toys[i].x- down[j].x, toys[i].y- down[j].y, up[j].x- down[j].x, up[j].y- down[j].y )< 0 )
            //if( (toys[i]- down[j])^ (up[j]- down[j]) )
            {
                cnt[j-1]++;
                break;
            }
        }
        for( int i=0;i<=n;i++ )
        {
            printf( "%d: %d\n", i, cnt[i] );
        }
    }
    return 0;
}